This study demonstrates the effectiveness of the KiVa antibullying program using a large sample of 8,237 youth from Grades 4-6 (10-12 years). Altogether, 78 schools were randomly assigned to intervention (39 schools, 4,207 students) and control conditions (39 schools, 4,030 students). Multilevel regression analyses revealed that after 9 months of implementation, the intervention had consistent beneficial effects on 7 of the 11 dependent variables, including self- and peer-reported victimization and self-reported bullying. The results indicate that the KiVa program is effective in reducing school bullying and victimization in Grades 4-6. Despite some evidence against school-based interventions, the results suggest that well-conceived school-based programs can reduce victimization. (Contains 6 tables.)